Merge pull request #937 from chkr1011/fix-writeasync-nre

Added checks for null references in the MqttTcpChannel.

Build/MQTTnet.nuspec

@@ -17,6 +17,7 @@
 * [Server] Adjusted some namespaces (BREAKING CHANGE!)
 * [Server] Added state checks (throw if not started etc.) for most server APIs.
 * [Server] Exposed real X509Certificate2 (instead byte array) to TLS options (thanks to @borigas).
+* [Core] Fixed a null reference exception in the MqttTcpChannel with WriteAsync and ReadAsync.
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2020</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin Blazor</tags> 

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -127,7 +127,14 @@ namespace MQTTnet.Implementations
                 // Workaround for: https://github.com/dotnet/corefx/issues/24430
                 using (cancellationToken.Register(Dispose))
                 {
-                    return await _stream.ReadAsync(buffer, offset, count, cancellationToken).ConfigureAwait(false);
+                    var stream = _stream;
+
+                    if (stream == null)
+                    {
+                        throw new ObjectDisposedException(nameof(stream));
+                    }
+
+                    return await stream.ReadAsync(buffer, offset, count, cancellationToken).ConfigureAwait(false);
                 }
             }
             catch (ObjectDisposedException)
@@ -157,7 +164,14 @@ namespace MQTTnet.Implementations
                 // Workaround for: https://github.com/dotnet/corefx/issues/24430
                 using (cancellationToken.Register(Dispose))
                 {
-                    await _stream.WriteAsync(buffer, offset, count, cancellationToken).ConfigureAwait(false);
+                    var stream = _stream;
+
+                    if (stream == null)
+                    {
+                        throw new ObjectDisposedException(nameof(stream));
+                    }
+
+                    await stream.WriteAsync(buffer, offset, count, cancellationToken).ConfigureAwait(false);
                 }
             }
             catch (ObjectDisposedException)